Lots of questions here. I went through this modification using the Harken 205. This is probably my favorite mods I have done to the boat... It makes it soooo much more fun to sail.
1. The swivel on it is orbital, or a controlled circle on a fixed plane. Follows you easily no matter where you move your "self ballast" to.
2. The ball bearings in the base make it very easy/smooth to rotate.
3. I now pull down on the sheet when I want it cleated, else I pull up to hold in hand. No more auto-cleating. This is great for hiking out on the rail, and making quick adjustments without having to lean back into an over powered boat in a gust. Basically, I feel I trim the sail more quickly/efficiently and take advantage of the gust vs missing opportunity by taking to much time trying to depower or depowering too much and having to shift through to many gears again.
I will tell you for certain on my '76 DSII (near same birthday as yours) with mid boom sheeting, there is a metal plate underneath the fiberglass the mainsheet swivel base is attached to. There are no nuts, the machine screws are threaded through the plate. I am pretty sure the fiberglass on the top of the centerboard trunk is pretty thick between 1/2" - 3/4". If you want an exact measurement of the thickness of the fiberglass, you could probably take one of the machine screws you pulled out and put it back in up until it touches the metal plate. Mark it with a sharpie marker, then back the machine screw out and measure from the tip of it to the mark you created on it. None the less, if you put it through the fiberglass and the metal plate... It ain't going anywhere, lol.
The holes for the Harken 205 do not match up to the factory holes for the original swivel base. I did not want to make it where things could not be returned to "stock", so I opted to put 4 new holes offset between the original holes. Another mental note, or more like insecurity, it that I would potentially push the plate off the bottom and then have to pull the centerboard to get into there to put the plate back. Thus I removed 3 of the 4 original machine screws, and loosed the last one. Rotated the original base out of the way of the other 3 holes, and then re-instaled the 3 screws I took out. Then finally removed the last screw and the original swivel base, and then reinstalled the final screw. Basically, at this point I know when I am pushing down on that drill to make the 4 new holes, the metal plate is secured by the original 4 screws and ain't going anywhere. Not sure if the plate actually would drop from putting pressure on it, but this exercise gave me piece of mind when it came time to break the drill out. And that metal plate was a real PITA to drill through. I ruined several screws trying to tap the holes and ruined an actual hole tapper (the official tool to do the job right).
Seeing how there was a metal plate under there, I wanted to make sure that I used machine screws (the ones with the flat tip, not pointed), so that they would be nicely threaded through the tapped plate. This of course makes the thread tapping process interesting with 1/2" - 3/4" fiberglass laying on top of that plate. But it is doable. I just started with the smallest drill bit and slowly worked my way up until I got a screw to start to catch. Then I would use the tapper (until I broke it) to finish up. 3 of the 4 holes went pretty smooth. The last one is where I feel like I spent forever and a day with the drill trying to get through, and this is also the hole that destroyed the tap. I eventually got it to tap with screws but went through a lot of them. Anyway, my point is buy extra screws.
Also, getting the holes in all the perfect locations was challenging with that thick base as well. It is hard to find a pen to get through the base and mark. I actually just marked the first hole, and then drilled it and tapped it. Then I put the base on being held by the one screw and then marked the other holes with the drill bit. This made sure I got perfectly aligned holes for the base.
This is probably a a lot of information, but hopefully gets you thinking about the silly little things that will pop up before you start. The project really was not that bad. I thought it would take 30 minutes, and it probably took 2 hours, lol. None the less I would do it 10 times over again if I had to. It was really really worth it.
My parts:
Harken 205
http://www.harken.com/productdetail.aspx?sku=205Harken Fiddle Carbo
http://www.harken.com/productcategory.aspx?taxid=422Harken 57mm Adjustable Auto Ratchet with Becket (if 3:1)
http://www.harken.com/productdetail.aspx?id=4741&taxid=421I use a 3:1 with the auto ratchet. Plenty of holding power and the sheet is very manageable. The way I look at it the auto ratchet takes care of sheeting out, making fine/small adjustments simple. The 3:1 takes care of sheeting in, giving enough resistance to allow me to not pull in to quickly (again making fine adjustments simple), but not over bearing where I cannot make a big adjustment easily if need be. I am a pretty little guy (150lbs) and left most of my muscle in high school, lol. The 3:1 with the ratchet is plenty of power for me personally.
Going to 4:1 or 5:1, I would not worry about stressing the boom, as the sheet just pulls the boom in. On the water, doing that is just going to make the boat heel. I would think the worry would be about overpowering your trim and losing the ability to make subtle/fine adjustments.